The present invention relates to spectral imaging and, more particularly, to a device and method for acquiring a spectral image of an object by illuminating the object successively, in a plurality of narrow spectral bands, by a plurality of narrow-band light sources, and capturing the image in each narrow band using additional optics such as a digital camera.
In spectral imaging, an image of an object to be studied is acquired at a series of discrete wavelengths, or, more generally, in a series of discrete narrow spectral bands. Two methods of accomplishing this are known, both of which illuminate the object with broad-band light. In the first method, narrow-band output is provided by filtering the light reflected or transmitted by the object, or by dispersing the light reflected or transmitted by the object using a dispersive optical element such as a prism or a diffraction grating. Images of the illuminated object are acquired successively in each spectral band that is passed by the successive filters or by the dispersive optical element. In the second method, the object is illuminated directly by a broad-band light source and, prior to imaging, the light from the object is passed through an interferometer which passes a particular linear combination of spectral bands, depending on the optical path differences (OPDs) of the interfering beams inside the interferometer. These OPDs are varied to obtain a complete set of linear combinations. The result is a set of acquired images of the object at the various linear combinations. Images of the object in the individual spectral bands are obtained from the acquired images by appropriate linear transformations. The second method is described, for example, in U.S. Pat. No. 5,539,517 to Cabib et al., which is incorporated by reference for all purposes as if fully set forth herein.
The relative efficiencies of these two methods have been compared by Felgett, in the context of (non-imaging) spectroscopy. See, for example, R. J. Bell, Introductory Fourier Transform Spectroscopy, Academic Press 1972, pp. 23-25 and J. Chamberlain, The Principles of Interferometric Spectroscopy, John Wiley & Sons 1979, pp. 305-306, which are incorporated by reference for all purposes as if fully set forth herein. The second method is the more efficient, because under the first method, most of the photons reflected or transmitted by the object are rejected, because of the narrow-band filtering, whereas under the second method, most or all of the photons reflected or transmitted by the object and passed by the interferometer and the imaging optics are collected.
Quantitatively, in the case of noise, such as random noise, that is independent of the signal level, the ratio of the signal-to-noise ratios of the two methods is proportional to the square root of the number of spectral bands: EQU R.sub.2 /R.sub.1 .about.M.sup.1/2 (1)
where R.sub.1 is the signal-to-noise ratio of the first method, R.sub.2 is the signal-to-noise ratio of the second method, and M is the number of spectral bands. In the case of noise proportional to the square root of the light source intensity (photon noise), both R.sub.1 and R.sub.2 obey EQU R.sub.j .about.[(T/M)I.delta..sigma.].sup.1/2 (2)
(j=1 or 2)
where T is the duration of the measurement and I .delta..sigma. is the source intensity in a spectral band of width .delta..sigma.. As a result, EQU R.sub.2 /R.sub.1 .about.1 (3)
In short, the second method is superior to the first method with respect to noise that is independent of signal level but not with respect to photon noise.
Nevertheless, the second method has certain disadvantages relative to the first method. First, the linear transformations needed to obtain narrow-band images are computationally intensive. A full set of narrow-band images must always be obtained, even if only certain bands are of interest. Second, the objects of interest include portions of live organs, such as the retina or fundus of the eye, which move during the course of the measurement, as the OPDs within the interferometer are varied. This makes it necessary to perform extensive processing of the acquired images before transforming them to narrow-band images, as described in co-pending U.S. patent application Ser. No. 08/942,122. Specifically, the images must be registered spatially, to compensate for the fact that the same pixel in different images does not generally correspond to the same part of a moving object. Furthermore, special Fourier Transform algorithms must be used that account for the fact that, after registration, the OPDs that define the interferograms are not uniformly spaced.
There is thus a widely recognized need for, and it would be highly advantageous to have, a spectral imaging method that would combine the advantages of presently known methods.